10 Mr. C. Barns on the Fusion 



latter is taken out of the muffle and submerged in water of 

 known temperature; but check-readings may be obtained 

 when the (cold) bulb is in place in the furnace, by inserting 

 the reservoir of a sensitive mercury thermometer into the tube 

 of the reentrant bulb, fig. 2. 



A special advantage of the furnace (which should always 

 be heavily jacketed with asbestos and banded with iron) is 

 the fact that the observer can get very near it without in- 

 convenience. Kotating parts may be lubricated. An elliptic 



Fig. 6. — Plan of the elliptic revolving muffle, with the air- 

 thermometers in place. Diagram. 



revolving muffle for the comparison of two gas- thermometers 

 has also suggested itself, particularly in view of the fact that 

 since the stem errors in this case vanish, comparisons may be 

 made with great accuracy. This is shown in figure 6, where 

 E C D F is the muffle and Om and Nn the two gas thermo- 

 meters to be compared. 



Thermoelectkic Apparatus. 



10. Wires and Insulators. — The thermocouples used con- 

 sisted of platinum combined with an alloy of platinum and 20 

 per cent, of iridium. The wires were cut in lengths of a metre 

 each, from large coils, and successive lengths showed the same 

 electric properties. Even when melted down on lime hearths 

 before the oxyhydrogen blowpipe and drawn over again, the 

 constants of the couples were not found to have appreciably 

 (t*o P or cen ^) changed. The wires should be annealed at red 

 heat before using. 



In order to keep the wires apart, I devised a machine by 

 which doubly perforated insulators could be pressed, much in 

 the manner of lead-pipe manufacture. The insulators were 

 made of very refractory fire-clay, 25 or 30 centim. long, and 

 •5 or '6 centim. thick, with two parallel canals, each '1 centim. 

 in diameter, running from end to end. 



11. Zero Method. — All my thermoelectric effects are mea- 

 sured, as electromotive force, in terms of a given Latimer 

 Clark's standard cell. The connexions made are easily 



